Looking to sign up with a service provider in order to do business in NYC as well as internationally. Which service is better for data? Which service is better for voice? Which is better internationally? Can anyone help???

I believe T-Mobile is bigger in Europe than Cingular, but it seems that most people using the phone internationally buy a local SIM. That's all I know from reading forums. For an extra $20/mo Cingular includes international roaming for data with the BB, not sure what T-Mobile offers. You might want to check at www.howardforums.com too.

As for NYC, I used T-Mobile for four years and I'm about a year on Cingular for voice. Hands down, in my experience, T-Mobile is better. Not a little better, a lot better. I use about 2200 min/mo with both carriers. Rarely does a day go by with Cingular where I either drop a call or get a fast busy (call failed) message. It is more of an annoyance than a problem. With T-Mobile, it is more like once a month.
T-Mobile has a lousy selection of phones, Cingular received the new BB six months before T-Mobile. Cingular is very slow to release software upgrades and is the only national carrier in the USA to have users on BWC rather than the newer BIS; T-Mobile had it back in October. Cingular lags far behind Verizon in 3g cell data, T-Mobile is even farther behind Cingular. T-Mobile has excellent customer service, Cingular is generally towards the bottom. T-Mobile is significantly less expensive, you can get a 3000 min/mo regional plan $50 + BB w/o BES for $20 for a total of $70/mo, get a calling card for those long distance calls. Cingular runs considerably more expensive. If you're in a city or major highway, T-Mobile has coverage but if you need service small towns Cingular has a much bigger network.